When presented with a fairly fresh zombie that still has attractive characteristics, the concept of sex with said zombie occurs to you and, depending on circumstances, may become reality.

When the zombie apocalypse comes, many people will have a wagonful of zombies held captive for just this purpose, and likely be prepared to kill living people just so they can have zombie sex with them.

Dude 1: Stacey is so hot, her bod is super sweet.

Dude 2: Forget it dude, the only way you'll ever fuck her is zombie sex.

When you and your partner both wake up in the middle of the night, and proceed to have sex. But you are soo tired it's very slow and mindless. Like a zombie.
Sometimes you may wake up and wonder if you actually had sex or not.

My girlfriend and I woke each other up and just started having zombie sex. It was so cool.
My fuck buddy slept over last night and then at some point in the middle of the night we both woke up so we just rolled over for some zombie sex.

While having sex with a girl, before you ejaculate you pull your dick out and ejaculate into her eyes. While she is unable to see you, you put hand cuffs on her feet to restrict her movement, then run away. Her blind restrictive movements will resemble those of a zombie.

Dude 1: Dude last night I had zombie sex, she wasn't happy at all, I left her like that and walked out of her apartment.
Dude 2: dude now she's just your zombie ex XD.